1. Field of the Invention
The present invention relates to a method of producing a semiconductor device which comprises a step of annealing, and in particular to a method of producing a semiconductor device which comprises a step of annealing an amorphous or polycrystalline silicon layer formed on a silicon dioxide layer.
2. Description of the Prior Art
It is often necessary to anneal defective crystals for the purpose of removing the defects induced by the ion implantation, or for the purpose of increasing the grain size of polycrystalline silicon. It is known that a beam applying process, for example a beam generated by means of a large output laser, has been used as an annealing process. For example, a yttrium Aluminum Garnet (hereinafter referred to as YAG) laser, ruby laser, argon laser, etc., which have a large optical absorption coefficient in an amorphous or polycrystalline silicon, have been used as the lasers for annealing of the ion implanted amorphous or polycrystalline silicon. There are two processes using a laser. One is pulse oscillation and the other is continuous wave (hereinafter referred to as CW) oscillation. The process using a YAG laser or ruby laser belongs to the former, and the process using an argon laser belongs to the latter. It is further known that a buried oxide metal oxide semiconductor (so called BOMOS) wherein the source, drain and gate of a MOS-FET are formed on a single crystalline layer, and the electrode and conductor pattern of the source and the drain are formed on a polycrystalline silicon layer, is superior to the usual bulk MOS device because BOMOS is able to reduce the junction capacitance between the source or drain region and the silicon substrate. In a structure wherein a silicon layer is deposited on a partially opened silicon dioxide layer which is formed on a single crystal silicon substrate, the silicon formed on the silicon dioxide layer may be a polycrystalline silicon layer and the silicon deposited on the single crystal silicon substrate has to be a single crystal silicon layer.
An example in which a conventional laser annealing process is applied to a structure in which a silicon dioxide layer and a polycrystalline silicon layer are successively formed on a single crystal silicon substrate will be explained with reference to FIG. 1. Referring to FIG. 1, a silicon dioxide layer 2 is formed on a single crystal silicon substrate 1 and further a polycrystalline silicon layer 3 is formed on the silicon dioxide layer 2. A laser beam (hereinafter referred to as the first laser) 5 is applied to the surface of the polycrystalline silicon layer 3 by reflecting it from a surface of a mirror 4 for laser annealing. Any laser of YAG or ruby or argon can be used as the first laser beam 5. When an annealing heat treatment is carried out by applying the laser beam 5 to the surface of the polycrystalline silicon layer 3 and thus heating it to a high temperature of from 1000.degree. C. to 1500.degree. C. for an instant, for example, in a range of ns (nanoseconds), the polycrystalline silicon layer 3 is melted and is rapidly cooled after the laser applying process is completed. In the rapid cooling a problem of thermal strain occurs due to the temperature difference of about 1000.degree. C. between the polycrystalline silicon layer 3 and the silicon dioxide layer 2. Defects, such as orange peel pattern, point defects, etc., may therefore be generated on the polycrystalline silicon layer 3. In order to reduce the above mentioned thermal strain, the single crystal substrate 1 is preliminarily heated to a temperature of ranging from 300.degree. to 500.degree. C. by means of a heater. However, when this preliminary heating is carried out, the advantages of the laser annealing technique using non thermal equilibrium process at a low temperature (i.e., high electrical activity, ease of handling, low possibility of contamination, etc.) are minimized. In the non-thermal equilibrium process, more impurities are electrically activated than in thermal equilibrium solid solubility. Therefore, the preliminary heating temperature of the single crystal silicon substrate 1 has a limitation and thus, the occurrence of the thermal strain between the polycrystalline silicon layer 3 and silicon dioxide layer 2 is not sufficiently prevented. The time necessary for the activation process, i.e., accepting implanted ions into the substitutional position in the substrate, is very short such as, from several hundreds ns to several .mu.s. Implanted ions have a tendency to possess a substitutional position with high proportion and have a high activation rate.
However, the ions are in a non equilibrium state when they are implanted and annealed by laser. When the single crystal layer 1 is heated to a high temperature by means of a heater, the ion-implanted and laser-annealed ions are present at the substitutional position or the interstitial position so that the ions are in thermal equilibrium. In this case the proportion of the substitutionals is not so high as the non-heated case. Since the interstitial ions are electrically nonactive, the activation rate of the implanted ions is, as a whole, lowered.